1. Field of the Invention
The present invention is generally directed to using multiple laser beams to heat, melt and bond metal members, and more particularly to laser bonding a metal member which has high absorption of energy from a first laser beam at the ambient temperature and high absorption of energy from a second laser beam after being heated by the first laser beam.
2. Description of Related Art
Lasers used in microelectronic bonding provide concentrated localized heating. This is especially useful for soldering or welding bonds on electrical interconnects such as customizable copper/polyimide substrates with fine pitch dimensions, low thermal stress tolerance or heat sensitive components.
Yttrium-aluminum-garnet (YAG) crystal doped with neodymium (Nd) can produce a laser beam with a fundamental wavelength of about 1064 nanometer (nm) wavelength which is infrared radiation and invisible to the human eye. Nd:YAG lasers provide a desirable balance between maximizing the energy absorption of metals and minimizing the energy absorption of polymer substrates. Nd:YAG laser outputs can be continuous-wave, shuttered with an acousto-optic or electro-optic device (Q-switch), or pulsed. While these outputs are each at 1064 nm wavelength, the peak output powers differ widely and these differences can have a profound effect on the suitability of any Nd:YAG laser for a particular application. Most currently available frequency doubled Nd:YAG lasers are either Q-switched or continuous-wave and as such are not suitable for microelectronic bonding. Continuous-wave frequency doubled Nd:YAG lasers produce an energy flux that is difficult to control and tend to thermally shock and damage a bond site. Q-switched frequency doubled Nd:YAG lasers produce extremely high peak power for short pulse widths, e.g. kilowatts for nanoseconds, and as such drill or cut through the electical members instead of bonding them by welding or solder reflow.
The use of 1064 nm wavelength Nd:YAG lasers for bonding electrical members of microelectronic components is well known in the art. A few related patents are as follows:
U.S. Pat. No. 4,697,061 to Spater et al. describes welding a metallic base to a thin, metallic covering which is highly reflective to the laser but covered with a metal skin which noticeably absorbs the laser.
U.S. Pat. No. 4,845,355 to Andrews et al. describes using a pulsed YAG laser to bond a bump on an integrated circuit to a tape-automated-bonding lead with a coating that has the property of being well coupled to the wavelength of the laser and which has a lower melting point than the melting point of the lead or bump.
U.S. Pat. No. 5,008,512 to Spletter et al. describes coating a coupling material on an electrical member and selecting the laser characteristics so that as bonding occurs an alloy of the electrical members solidifies and a solidification front drives the molten coating and molten compounds containing the coating away from the bond interface towards the exterior periphery of the bond. As a result, substantially all of the solidified bond interface consists of an alloy of the electrical members and substantially all of the bond interface strength results from the alloy of the electrical members.
U.S. Pat. No. 5,049,718 to Spletter et al. is similar to the '512 except that instead of coating the coupling material on an electrical member, the electrical member is coated with a coating alloy which comprises the coupling material and the same metal as the other electrical member.
Each of the foregoing patents describes bonding the metal members by using a single laser beam, usually a 1064 nm wavelength Nd:YAG. Frequency doubled pulsed Nd:YAG lasers create green light at 532 nm wavelength (sometimes rounded to 533 nm) which is visible to the human eye and often used for rangefinding, night vision, and heating large metal structures not found in microelectronic bonding. Commercial vendors include Lumonics Corp. and Kigre Corp.
Additional laser beams have been used in bonding operations for purposes other than supplying energy to heat the bond. For example, U.S. Pat. No. 4,845,354 to Gupta et al. describes a process control for laser wire bonding in which a low power laser beam which is co-linear with a high power laser beam is conducted to the bond site and reflected from the wire during the bonding cycle. The change in reflectivity of the wire is detected by the low power laser beam and a signal commensurate with the detected change of reflectivity is used to control the pow.RTM.r or duration of the high power laser beam during bonding.
Nevertheless, lasers at shorter wavelengths in the range of 300-600 nm, in particular the Nd:YAG frequency doubled 532 nm wavelength provides a substantial increase in the amount of laser energy absorbed by appropriate metal electrical members as they are heated and bonded, as pointed out by M. Greenstein in "Optical Absorption Aspects of Laser Soldering for High Density Interconnects," Applied Optics, Vol 28, No. 21, Nov. 1, 1989, pp. 4595-4603 which is incorporated herein by reference. The article reports that for both gold and copper metallurgies the 532 nm wavelenqth of a Nd:YAG laser provides significantly more absorption of laser energy than the 1064 nm wavelength. Furthermore, as the temperature of gold and copper increases the predicted optical absorption of gold and copper, respectively, increases as well. For example, when a frequency doubled pulsed Nd:YAG laser beam was directed at gold plated copper surfaces of tape-automated-bonding lead frames, 40% of the energy from the green component was absorbed versus about 1-5% of the energy from the infrared component.
Building on the work of Greenstein, a frequency doubled pulsed laser beam for bonding electrical members is described in U.S. application Ser. No. 07/561,555 by Spletter et al., filed Aug. 1, 1990, which is the parent of the instant application, assigned to the assignee of the present invention and incorporated herein by reference. The '555 also describes a method of adjusting the mix of base and frequency doubled wavelengths in the laser beam. Perhaps most importantly, the '555 provides for laser bonding highly reflective members such as gold and copper without any absorbent coupling material such as tin by using a frequency doubled pulsed Nd:YAG laser beam so that the 532 nm wavelength heats the metal members until their temperature increases, their reflectivity decreases and they become absorbent to the 1064 nm wavelength. The 1064 nm wavelength further heats and eventually melts the members so that a solid alloy bond is formed therebetween.